The Collaborative International Dictionary of English v.0.48:

Trigonia \Tri*go"ni*a\, n. [NL. See Trigon. So called in
   allusion to the triangular shape of some species.] (Zool.)
   A genus of pearly bivalve shells, numerous extinct species of
   which are characteristic of the Mesozoic rocks. A few living
   species exist on the coast of Australia.
   [1913 Webster]